template="Write a catchphrase for the following company: {company_name}",
)
chain_two = LLMChain(llm=llm, prompt=second_prompt)
Now we can combine the two LLMChains, so that we can create a company name and a catchphrase in a single step.
from langchain.chains import SimpleSequentialChain
overall_chain = SimpleSequentialChain(chains=[chain, chain_two], verbose=True)
# Run the chain specifying only the input variable for the first chain.
catchphrase = overall_chain.run("colorful socks")
print(catchphrase)
> Entering new SimpleSequentialChain chain...
Cheerful Toes.
"Spread smiles from your toes!"
> Finished SimpleSequentialChain chain.
"Spread smiles from your toes!"
Create a custom chain with the Chain class#
LangChain provides many chains out of the box, but sometimes you may want to create a custom chain for your specific use case. For this example, we will create a custom chain that concatenates the outputs of 2 LLMChains.
In order to create a custom chain:
Start by subclassing the Chain class,
Fill out the input_keys and output_keys properties,
Add the _call method that shows how to execute the chain.
These steps are demonstrated in the example below:
from langchain.chains import LLMChain
from langchain.chains.base import Chain
from typing import Dict, List
class ConcatenateChain(Chain):
    chain_1: LLMChain
    chain_2: LLMChain
    @property
    def input_keys(self) -> List[str]:
        # Union of the input keys of the two chains.
        all_input_vars = set(self.chain_1.input_keys).union(set(self.chain_2.input_keys))
        return list(all_input_vars)
    @property